Car radio receivers need to cope with changing strengths of received radio signals from different radio channels on different receivers as cars drive around. The car can reach areas where the signal received fades. In these cases, it is useful for the radio to find other transmitters transmitting the same channel and to automatically retune.
One way of doing this is known as the Radio Data System (RDS). Radios equipped with RDS receive data together with the audio signal, the data being transmitted at 57 kHz. The data includes information allowing retuning.
It would be desirable to be able to carry out the same retuning without requiring RDS. In order to carry out the retuning, it is necessary to identify other channels being received carrying the same signal as the channel presently being listened to, in other words comparing audio streams.
Systems for comparing audio streams are known. A particular example is the automatic identification of tracks of recorded music stored on a player. Algorithms exist for identifying the tracks of recorded music by comparing features of the tracks with a large database.
However, such systems are not suitable for use with car radios since the car radio environment has a number of difficult features. Firstly, there can be relatively significant time shifts between different transmitters, certainly as much as 100 ms. Secondly, the audio transmitted by radios can be very heavily compressed. Thirdly, there can be considerable signal distortion.
A further problem is that the systems must work in real time in car radios. Even digital car radios have limited computer processing power. Moreover, there is no large pre-prepared database with which to compare the audio channels.
A particular problem with identification of new channels in a retuning application is that the method must work at the time the existing signal is starting to fade. In this case, there can be significant distortion and also some fading of the audio signal. An audio correlation system for use in this environment must therefore be robust.